The present invention relates in general to magnetic resonance imaging (magnetic resonance tomography) as applied in medicine for examining patients. The present invention relates, in particular, to a magnetic resonance imaging apparatus and to a method for operating such an apparatus, of the type employing an EPI pulse sequence to obtain a spatially resolved navigator rod on the longitudinal axis of the body.
Magnetic resonance imaging is a tomographic method for medicine diagnostics which is distinguished first and foremost by a high contrast resolving power. Because of its excellent ability to display soft tissue, magnetic resonance imaging has developed into a method greatly superior to X-ray computed tomography. Magnetic resonance imaging is currently based on the application of spin echo and gradient echo sequences which permit an excellent image quality in conjunction with measuring times of the order of magnitude of minutes.
In many fields of medical diagnostics, it is of interest to display physiological processes by means of magnetic resonance images. It is necessary for this purpose to synchronize these physiological processes with the appropriate magnetic resonance methods.
A typical application is the monitoring of the position of the heart when obtaining heart images. Obtaining the position information is facilitated, for example, by a profile of the spin density in a rod on the longitudinal axis of the body, a so-called navigator rod.
In known techniques, a navigator rod is usually obtained by means of a spin echo sequence whose selective 90xc2x0 and 180xc2x0 RF pulses have respective slice planes that intersect as a cross. The spin echo is therefore produced exclusively in the intersection region of the two slice planes.
However, this method has the significant disadvantage that the longitudinal magnetization in the two slices, as well as in the intersection region with the measuring volume, is saturated or inverted. The orientations of the two planes therefore must be selected such that they do not intersect the volume of the following image measurement.
Apart from difficult manipulation, this has the consequence that, for example, the movement of the heart due to breathing cannot be measured directly, but must be derived from the movement of the more effectively accessible other half of the diaphragm.
It is an object of the present invention to provide a magnetic resonance apparatus and method which permit a navigator rod to be obtained without impairing the measuring volume for the subsequent measurement.
This object is achieved in accordance with the invention in a magnetic resonance apparatus which has a system for generating an EPI pulse sequence which irradiates an object to be examined. Excitation, which is selective by means of a slice selection gradient which has a small flip angle xcex1 less than 90xc2x0, produces a signal which is read out using phase encoding in a direction perpendicular to the slice selection direction and read out direction and is used to obtain a spatially resolved navigator rod.
The navigator slice is usually arranged parallel to a measuring slice used in a subsequent measurement.
The oscillating read out gradient has positive and negative gradient lobes, the echoes of both gradient lobes or of only one lobe being used.
The signals of a column/row of the navigator slice fixed by the slice selection direction are added by weighted addition to form an aggregate echo from which the navigator rod is calculated.
In accordance with the invention and a method for operating a magnetic resonance imaging apparatus wherein nuclear spins are excited with an excitation pulse having a small flip angle which is less than 90xc2x0, activating a slice selection gradient, activating a phase encoding gradient, generating multiple gradient echoes with an oscillating read out gradient, thereby obtaining a set of gradient echoes following the excitation pulse, and generating a spatially resolved navigator rod from a signal from the selective excitation with the small flip angle of less than 90xc2x0, this signal being read out with phase encoding in a direction perpendicular to the slice selection direction and perpendicular to the read out direction.